Cycles to Failure

The fatigue strength of lead has a frequency dependence [184]. The stress fluctuations in cable sheathing and outdoor installations frequently arise from temperature changes that occur over the day, and frequencies commonly encountered [239] will have -I cycles/day as the lowest frequency. Another source of thennal stress is the variations in current in the case of high-voltage power transmission cables. Fatigue behavior of Pb and the Pb-I wt.% Sb alloy at frequencies of 0.25 per minute and 1650 per minute are compared in Figure 81 [239]. At an alternating strain of ±O.2%,

I"'" ~ ksf with rest intel'YOls in ail :--- 1\1

' ....~ "r-2~'O+ 2 •...5 oJ

Figure 80 S-N curves for lead in air and in vacuum [2,231]. (Courtesy of Springer Verlag, New York.)

Table 40 Effect of Surrounding Media and Protective Coatings on the Fatigue Resistance of Lead and Lead Alloys [2.238]. (Courtesy of Springer Verlag. New York.)

Materials

Pb

Pb + 1.5% Sn + 0.25% Cd

Pb + 0.5% Sb + 0.25% Cd

Surrounding medium or

protective coating

Air Normal acetic acid Rape oil Vaselin Air Petroleam bitumen Air Rape oil Vaselin

Semirange of stress (:!:MPa)

0.54 0.54 0.54 0.62 1.0 1.2 1.2 1.4 1.4

Endurance cycles (l<t)

1.3 8.5 U" 7.9 9.8 U 1.6 9.3 U 1.3 9.6 6.4

''I

Figure 81 Effect of frequency on number of cycles to failure [2.239]. (Courtesy of Springer Verlag. New York.)